Many genetic traits which are of greatest medical or economic importance are complex quantitative traits, such as weight, blood pressure, or susceptibility to disease. Such traits are usually affected by more than one gene. They are also usually affected by non-genetic factors such as the environment. They are typically described as a numerical value rather than as a set of descriptive names. In recent years, it has become feasible to identify many of the individual genes which contribute to quantitative traits. These genes are known as quantitative trait loci. The long-term goal of this proposal is to create and distribute software which will aid the identification and characterization of these genes in humans and other organisms. This software will be portable, user- friendly, well-documented microcomputer software to identify and map quantitative trait loci in experimental animals and plants. To develop this software, we propose to add functions for mapping quantitative trait loci to an existing portable genetic mapping program, Map Manager XP. Once developed, this software and its documentation will be distributed without charge to any laboratory requesting them.